As I understand it, what seemed to be specific to Red Hat and CentOS distributions seems to also be happening on Ubuntu and Debian systems, as well. So…don’t upgrade your kernel and grub until the upstream vendors sort out the issue.
I know Virtualmin offers to update any available packages, but it’s not us recommending you do so, it’s just Virtualmin informing you about available updates (and normally we do recommend you stay on top of updates for security reasons…sometimes updates break things, though rarely this badly, but the risk of not updating regularly is usually much higher). But, please stop blaming us. We didn’t break your system, your OS provider did. (But don’t blame them, either, they were trying to protect you from a serious security bug.)
UPDATE 8/15/20: I’m reasonably confident this has been fixed everywhere. I’ve done numerous upgrades spread across CentOS and Ubuntu versions (and Fedora, though that doesn’t matter for most Virtualmin users) using UEFI boots and they’ve been working fine for the past week. I don’t have links to changelogs, but I’m sure more documentation exists. I think it’s safe to just assume this whole sordid episode is behind us and get back to regularly updating our systems as though it were a religion.